[Bug]: Potential Out-of-bounds in cache_kernels.cu

[molly-ting]

Your current environment

The output of python collect_env.py

Your output of `python collect_env.py` here

🐛 Describe the bug

While performing static analysis on CUDA kernels, I identified potential out-of-bounds accesses in the gather_and_maybe_dequant_cache kernel in cache_kernels.cu.
1. batch_block_table[pid]

vllm/csrc/cache_kernels.cu

Lines 968 to 969 in a00d625

	for (int pid = split_start; pid < full_blocks_end; ++pid) {
	auto block_id = batch_block_table[pid];

batch_block_table[pid] may lead to an out-of-bounds access.
block_table has shape: [1, u0]

index = batch_offset + offset + pid
      = blockIdx.x * block_table_stride + seq_starts[bid] / block_size + pid
      = blockIdx.x * u0 + seq_starts[blockIdx.x] / 64 + pid

Example Scenario

batch_block_table.shape: [1, 2] 
blockIdx.x: 0
blockIdx.y: 0 
seq_starts[0]: 128 
pid: 0

Under these conditions, batch_block_table[pid] accesses invalid memory due to an out-of-bounds index.
2. batch_block_table[full_blocks_end]

vllm/csrc/cache_kernels.cu

Lines 978 to 979 in a00d625

	if (partial_block_size) {
	auto block_id = batch_block_table[full_blocks_end];

Similarly, batch_block_table[full_blocks_end] may also lead to an out-of-bounds access.
Example Scenario

batch_block_table.shape: [1, 2] 
blockIdx.x: 0
blockIdx.y: 0 
seq_starts[0]: 128 
cu_seq_lens[1]: 1
cu_seq_lens[0]: 0
pid: 0

In this case, batch_block_table[full_blocks_end] also accesses invalid memory.

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